Recently, metasurfaces have gained popularity due to their ability to offer a spatially varying phase response, low intrinsic losses and high transmittance. Here, we demonstrate numerically and experimentally a silicon meta-surface at THz frequencies that converts a Gaussian beam into a Vortex beam independent of the polarization of the incident beam. The metasurface consists of an array of sub-wavelength silicon cross resonators made of a high refractive index material on substrates such as sapphire and CaF2 that are transparent at IR-THz spectral
range. With these substrates, it is possible to create phase elements for a specific spectral range including at the molecular finger printing around 10 μm as well as at longer THz wavelengths where secondary molecular structures can be revealed. This device offers high transmittance and a phase coverage of 0 to 2π. The transmittance phase is tuned by varying the dimensions of the meta-atoms. To demonstrate wavefront engineering, we used a discretized spiraling phase profile to convert the incident Gaussian beam to vortex beam. To realize this, we
divided the metasurface surface into eight angular sectors and chose eight different dimensions for the crosses providing successive phase shifts spaced by π/4 radians for each of these sectors. Photolithography and reactive ion etching (RIE) were used to fabricate these silicon crosses as the dimensions of these cylinders range up to few hundreds of micrometers. Large 1-cm-diameter optical elements were successfully fabricated and characterised by optical profilometry.

We describe a systematic approach to design, optimize, and characterize a Fourier-domain mode-locked (FDML) laser with an erbium-doped fiber amplifier (EDFA) as the optical gain medium. A highly stable temporal intensity profile is obtained by minimizing chromatic dispersion and polarization fluctuations. The obtained bandwidth of 21 nm, tuning speed of 50 kHz, and output power of 5 mW are the highest reported so far with an EDFA-based FDML laser.

Subwavelength gratings exhibit attractive polarizing properties and have promising applications in communication, optical information processing, holography, and displays. The fabrication of subwavelength binary gratings for operation as polarizing beam splitters (PBS) at a wavelength of 1550 nm is presented. A simplified modal method was used for the design as well as to predict the efficiencies of the polarization components in each order. Electron beam lithography has been employed for patterning subwavelength grating structures on polymethyl methacrylate (PMMA) resist. The fixed beam moving stage patterning mode is used for patterning gratings with a period of 936 nm and width of 374 nm. The exposure and developing parameters are optimized to realize the grating with the designed feature sizes on PMMA resist. Gratings patterned using the optimized exposure and development parameters match well with the design, except for the height. The performance of the fabricated PBS grating has been evaluated by optical testing. The experimental results match well with the predictions.

We present an electrically actuated tunable optical grating based on dielectric elastomer actuators. A simple fabrication protocol is presented, which integrates the grating with the actuating mechanism both made of soft elastomers, improving the tunability of the grating. The device is designed to be operated in the transmission mode. It exhibits a continuous period tunability of 34.4% at an actuation voltage of 5.5 kV, which is an improvement over reported tunable optical gratings.

A binary Fresnel Zone Axilens (FZA) is designed for the infinite conjugate mode and the phase profile of a refractive axicon is combined with it to generate a composite Diffractive Optical Element (DOE). The FZA designed for two focal lengths generates a line focus along the propagation direction extending between the two focal planes. The ring pattern generated by the axicon is focused through this distance and the radius of the ring depends on the propagation distance. Hence, the radius of the focused ring pattern can be tuned, during the design process, within the two focal planes. The integration of the two functions was carried out by shifting the location of zones of FZA with respect to the phase profile of the refractive axicon resulting in a binary composite DOE. The FZAs and axicons were designed for different focal depth values and base angles respectively, in order to achieve different ring radii within the focal depth of each element. The elements were simulated using scalar diffraction formula and their focusing characteristics were analyzed. The DOEs were fabricated using electron beam direct writing and evaluated using a fiber coupled diode laser. The tunable ring patterns generated by the DOEs have prospective applications in microdrilling as well as microfabrication of circular diffractive and refractive optical elements.

The phase of a negative axicon is combined with that of a Fresnel zone lens (FZL) to obtain an element labelled as
conical FZL, which can generate a focused ring pattern at the focal plane of the FZL. The phase integration is achieved
by modifying the location and width of zones of FZL in accordance with the phase variation of the negative axicon. The
element was designed for a high power laser with a wavelength of 1064 nm, focal length and diameter of conical FZL of
30 mm and 8 mm respectively and for a ring diameter of 50 μm. The element was fabricated using photolithography. The
pattern was transferred from the resist layer to the borosilicate glass plates by dry etching to achieve an etch depth of
1064 nm. The etch depth measured using confocal microscope was 1034 nm at the central part and 930 nm for the
outermost part of the device with a maximum error of 12.5% at the outermost part and 3% at the central part. The
element was used in an optical trapping experiment. The ring pattern generated by the conical FZL was reimaged into the
trapping plane using a tightly focusing microscopic objective. Polystyrene beads with diameters of 3 μm were
suspended in deionized distilled water at the trapping plane. The element was found to trap multiple particles in to the
same trap.

The design and fabrication of transmission subwavelength binary gratings for operation as polarizing beam splitters
(PBSs) at 1550 nm is presented in this paper. An analytical method called the modal method was used for the design as
well as to predict the efficiencies of the polarization components in each order. Electron beam lithography has been
employed to fabricate the subwavelength grating structures on poly methyl methacrylate (PMMA). The performance of
the fabricated PBS has been evaluated by optical testing.

Focused ring patterns are used for many applications like corneal surgery, micro drilling, optical trapping, etc. The generation of focused ring patterns in the earlier reported cases employed many refractive optical components with different functions. As a result the optics configurations of the ring pattern generation systems are bulkier. In diffractive optics, it is possible to alter a function of an element and also integrate multiple functions in a single element. In this paper, we present the design, fabrication and evaluation of single and composite diffractive optical elements for the generation of focused ring patterns. A diffractive toric Fresnel zone lens was designed for parallel beam illumination. This element is compared with other composite diffractive elements capable of generating focused ring patterns. The toric Fresnel zone lens and composite elements were fabricated using electron beam direct writing. The fabricated elements were found to exhibit interesting properties, with the toric lens out-performing the other elements in several areas such as efficiency, focal depth, and ring thickness.

Diffractive optics has traditionally been used to transform a parallel beam of light into a pattern with a desired phase and intensity distribution. One of the advantages of using diffractive optics is the fact that multiple functions can be integrated into one element. Although, in theory, several functions can be combined, the efficiency is reduced with each added function. Also, depending on the nature of each function, feature sizes could get finer. Optical lithography with its 1 μm limit becomes inadequate for fabrication and sophisticated tools such as e-beam lithography and focused ion beam milling are required. Two different techniques, namely, a modulo-2π phase addition technique and an analog technique for design and fabrication of composite elements are studied. A comparison of the beams generated in both cases is presented. In order to be able to compare methods, specific functions of ring generation and focusing have been added in all cases.

Higher Order Bessel Beams (HOBBs) have many useful applications in optical trapping experiments. The generation of
HOBBs is achieved by illuminating an axicon by a Laguerre-Gaussian beam generated by a spiral phase plate. It can also
be generated by a Holographic Optical Element (HOE) containing the functions of the Spiral Phase Plate (SPP) and an
axicon. However the HOBB’s large focal depth reduces the intensity at each plane. In this paper, we propose a multifunctional
Diffractive Optical Element (DOE) containing the functions of a SPP, axicon and a Fresnel Zone Lens (FZL)
to generate higher efficiency higher order Bessel-like-beams with a reduced focal depth. The functions of a SPP and a
FZL were combined by shifting the location of zones of FZL in a spiral fashion. The resulting element is combined with
an axicon by modulo-2π phase addition technique. The final composite element contains the functions of SPP, FZL and
axicon. The elements were designed with different topological charges and fabricated using electron beam direct writing.
The elements were tested and the generation of a higher order Bessel-like-beams is confirmed. Besides, the elements also
generated high quality donut beams at two planes equidistant from the focal plane of the FZL.

Diffractive optics has traditionally been used to transform a parallel beam of light into a pattern with a desired phase and intensity distribution. One of the advantages of using diffractive optics is the fact that multiple functions can be integrated into one element. Although, in theory several functions can be combined, the efficiency reduces with each added function. Also, depending on the nature of each function, feature sizes could get finer. Optical lithography with its 1 μm limit becomes inadequate for fabrication and sophisticated tools such as e-beam lithography and focused ion beam milling are required. In this paper, two different techniques of fabrication of composite elements are studied. A comparison of the beams generated in both cases is presented.

In the past, UV lithography has been used extensively for the fabrication of diffractive optical elements (DOEs). The advantage of this technique is that the entire structure can be written at one time, however, the minimum feature size is limited to about 1 μm. Many 1-d and 2-d periodic grating structures may not need such fine details but it is essential for diffractive optics with circular structures. This is because the spacing between features typically decreases towards the edge of the element resulting in the smallest feature falling well below 1 μm. 1-d structures such as sub-wavelength gratings will also have smaller feature sizes throughout the structure. In such cases, advanced techniques such as Focused Ion Beam and Electron-beam Lithography are required for the fabrication of finer structures. In this paper, we present results of DOEs fabricated with a focused ion beam system (Nova Nanolab 600 from FEI) directly on a single mode fibre tip. The ability to write DOEs directly on fibre tip is of great importance in fields such as endoscopy and optical trapping. The DOE itself, transforms the laser beam to a phase and intensity profile that matches the requirement. Because it is located directly on the fibre, no extra alignment is required. In addition, the system becomes more compact, which is especially important for applications in the field of endoscopy. The main goal of the present work was to develop the most accurate method for creating the desired pattern (that is, the DOE structure) into an actually working element. Different exposure strategies for writing test structures directly with the ion beam on the fibre tip have been tested and carefully evaluated. The paper will present in detail the initial fabrication and optical test results for blazed and binary structures of 1-d and circularly symmetric Fresnel axicons on optical fibres.

Sub-wavelength dielectric gratings can be used to achieve phase retardation. Due to the vector nature of the devices, scalar theory is not applicable and rigorous calculation methods are required. The modal method proves to be a simple but powerful compromise, between rigorous techniques that are computationally expensive and the scalar theory that is inadequate, for design of such elements. As a proof of concept, a quarter wave plate (QWP) was designed and its behaviour compared against previously published data. Wave plate design requires that the orthogonal polarizations travel in the same direction with appropriate phase delay. It is assumed that light is incident normally on the grating. Floquet-Bloch periodicity ensures that discrete modes get excited within the grating. The number of propagating modes and the propagation constant of the modes can be controlled by the angle of incidence, the ratio of period to the incident wavelength and the fill factor. Modal method characterizes the underlying Eigen modes (/ effective indices) of the orthogonal field components in the sub-wavelength structure. Based on the indices obtained by modal method, height of the grating ridge is deduced. The design gives a high aspect ratio of about 8 for a quarter wave phase retarder. The design is also numerically evaluated by the finite element method. The solver COMSOL was used to visualize how polarization direction evolves with time. The designed QWP could convert linearly polarized light into circularly polarized light and vice versa. This result proves the validity of the design procedure.

This article [J. Micro/Nanolith. MEMS MOEMS. , 12, (4 ), 041203 (2013)] was originally published online on 25 September 2013 with errors in Figure 6. The figure sequence in the caption did not match the sequence given in the figure. Also, Fig. 6(d) showed an image of aluminum mask with a crossbar on the image which is not visible and an improper scale with a typographical error.
The corrected figure and caption appear below. Online versions were corrected on 08 January 2014.

The angles of diffraction of a grating are a function of the incident wavelength and the period of the grating. The ability to vary the period of a grating is of interest in a variety of applications, as this in effect provides tunability of the diffraction angles. Most tunable gratings vary the period in an analog fashion. This is achieved by pulling the grating beams apart, which varies the gap between the beams, and hence the period. Changing the period in this way usually does not alter the width of each beam. Although this provides tunability with a high resolution, the ratio of the width of each grating beam to the period changes for each period. This in turn changes the efficiency for each grating period. In this work, we present a design of a MOEMS grating that changes the period between two values, i.e., from 12 to 24 μm and 48 μm, while keeping the efficiency constant. Electrostatic attraction is used to selectively pull down beams in order to achieve this. Simulation and experimental results show a variation of about 1% in efficiency between the two grating states. The measured efficiencies in the first order of the 24- and 48-μm period gratings were found to be 6.5% and 7% compared to the simulated values of 9% and 10%, respectively. The gratings were fabricated using surface micromachining. The process flow is presented.

GaN light emitting diodes (LEDs) on sapphire substrates can be improved by micro-patterning substrate to perform epitaxial over-growth which drastically reduces defects' density in the light emitting region. We patterned Al2O3 with focused ion beam and show a successful overgrowth of GaN. The exact shape of pattern milled into Al2O3 was replicated into a 0.4-mm-thick shim of Ni by electroplating. The surface roughness of Ni was ~5:5 ±2 nm and is applicable for the most demanding replication of nano-rough surfaces. This technique can be used to replicate at micro-optical elements Fresnel-axicons defined by electron beam lithography made on sub-1 mm areas without stitching errors (Raith EBL). Shimming of macro-optical elements such as car back- reflectors is also demonstrated. Ni-shimming opens possibility to make replicas of nano-textured small and large area patterns and use them for thermal embossing and molding of optically-functionalized micro-fluidic chips and macro-optical elements.

Conventional photolithography normally utilizes a photomask for patterning light onto a chemical resist film. Therefore, the accuracy of microfabrication is highly dependent on the accuracy of the photomasks. Fabrication of hard masks involves the use of expensive laser pattern generators and other sophisticated machines using very high-precision stages and the necessary control instrumentation; therefore, an inexpensive strategy is highly necessary for laboratory-level fabrication. As this technology is primarily based on raster scanning of a laser beam, the mask making as such becomes a low-throughput process. A strategy of high-throughput manufacturing of hard masks with laser micromachining using a one-step exposure process of a chromated glass slide through a micromachined aluminum shadow mask is proposed. The features that are finally embedded in the mask are highly demagnified and well focused. Optimization of the laser machining process is carried out by considering all processing parameters. The features are characterized using an optical microscope, a scanning electron microscope, and a self-developed image analysis code. Geometrical methods are used to estimate the average edge roughness and feature size. We have also validated the usage of these masks by performing microfabrication on films made of photoresist.

We report on improvements in speckle tracking algorithms for analyzing subsurface vibrations using an optical coherence tomography system. Our technique uses axial deconvolution and linear interpolation of depth scans before using cross-correlation-based speckle tracking. We applied this method on M-mode images of a test sample and on wrist pulses to perform depth-resolved measurements of displacement, velocity, and frequencies of vibrations. Speckle pattern displacements and features spanning 0.5 pixels were tracked clearly. Extension of this technique to two dimensions can provide improved performance in the field of optical coherence elastography.

MEMS diffraction gratings whose period can be tuned during operation find applications in microspectrometers and in display devices. These structures were designed using fixed-fixed beams of 10 μm width and spacing of 2 μm between them and were actuated in an out-of-plane motion. It was found that non-planar bending of fixed-fixed beams reduces the amount of light diffracted into the first order by about 4% compared to ideal planar beam displacement. A new fixed-fixed beam design with modified spring constant and actuation mechanism was proposed to improve the planarity of the beam during actuation. The profiles of the two structures (proposed design and normal beam), when displaced by λ/4 (relative displacement between two beams required for diffraction), were extracted using Intellisuite simulations and this data was fitted with a ninth order polynomial to model the beam displacement. The grating transfer function was modeled with the acquired displacement profiles and it was found that the new design improved the efficiency in the first order by about 2% when simulated in MATLAB for 633 nm wavelength. The grating structures were fabricated using a 4-mask process and were released using a wet release process developed in the lab. The electrical characterization of the devices indicates that the devices were released. The grating structures were tested optically using an optical setup built in the lab.

Tunable gratings find applications in spectrometers, tunable cavity lasers and projection displays. In this paper,
we discuss the design, modeling, fabrication process and optical characterization of a reflective type period
tunable MEMS diffraction grating. The optical modeling of different grating periods for optimum efficiency is
presented. A new fixed-fixed beam design with a uniform displacement over 33% of its length is proposed and
simulated. The grating membranes are fabricated with gold metal. The optical characterization of the fabricated
structure is presented.

We present a method for mitigating space variant blur occurring in the images acquired using Optical coherence
tomography (OCT). The effect of Gaussian beam divergence on the image resolution is analyzed mathematically
to develop space dependent two dimensional point spread functions that define the blurring kernel. Two standard
deconvolution algorithms are used to deblur the images using the space dependent point spread functions. We
show that the deconvolution method is effective in improving the transverse resolution of cross sectional OCT
images at regions up to several times as deep as the confocal region of the Gaussian beam.

A fibre based Optical Coherence Tomography (OCT) system has been developed and characterized. A Micro Electro
Mechanical System (MEMS) mirror was used to scan infrared beam over the sample. In this paper, we have presented
the system, its specifications and the results obtained. In particular we discuss about the suitability of the MEMS mirror
based OCT to image vibrating samples.

Fractal Zone plates and optical pin sieves are designed and their focusing properties are analyzed using scalar diffraction formula. It is found that these zone plates offer better control on beam parameters compared to Fresnel Zone plates. These zone plates are then fabricated on to quartz substrates using UV lithography and the results are confirmed.

This paper presents a design procedure for a fibre interferometer, the optical system and its associated electronic
control. Analog and digital circuits were optimized to achieve a low cost compact system. The lock-in amplifier
required for phase control was designed using a FPGA. The errors in an interferometric measurement were
studied in detail and its results were used to estimate the capabilities of the interferometer. These matched
our observed resolution of 40 nm. A stabilization technique of controlling the path length difference between
the arms of interferometer nullifies any phase errors. The design and testing of the control circuit are described
in detail. In addition, the FPGA was programmed to carry out phase stepping, as this technique is used to
calculate the desired phase. The interferometer was used to measure samples with step heights in the hundreds
of nanometers, with improvements in accuracy through averaging of data. We verified the successful working of
the instrument by measuring a height of 423 nm for a 420 nm structure.

A wide variety of MEMS micro-mirrors are being developed for various optical applications. One such application is Fourier Transform Spectrometry (FTS). The design, process optimization and fabrication of a micro-mirror for this application is presented. Large, non-tilting displacements of mirrors are required to achieve high FTS resolution. In order to obtain this without using Deep Reactive Ion Etching (DRIE), the micro-mirrors were fabricated on silicon using bulk micromachining techniques. This paper will present the process developed for fabrication of the mirror with the required specifications. In addition, results of the FTS experiments conducted with the micro-mirror will also be presented.

The interdisciplinary nature of optical microelectromechanical systems (MEMS) makes its design a highly involved process. To design a device that will meet the required specifications over its entire lifetime, reliability issues must be included in the design process. This work develops a method that can be used to create a step-by-step design process involving reliability issues at every stage. The process begins by listing out the constants and constraints of the design. Taking all such constraints into account, an optical design process is outlined. The effect of individual errors is studied on a key parameter like insertion loss. This gives the designer information about which parameters the design is more sensitive to, and will help in deciding the manufacturing tolerances required in the different stages. Using all of these data, an "ideal design" is developed. A Monte Carlo analysis is carried out on that design to show the effect of errors occurring simultaneously. The work concludes with a flow chart of a suggested design process to be used when designing optical MEMS devices.

The inter-disciplinary nature of MEMS makes its design a highly involved process. In Optical MEMS the addition of optical parameters only increases the complexity. It is almost impossible for a designer to know where to start given the number of variables involved. In order to design a device that will meet the required specifications over its entire lifetime, reliability issues must be included in the design process. This paper develops a method that can be used to create a step-by-step design process involving reliability issues at every stage. The process begins by listing out the constants and constraints of the design. The constraints could be due to physical parameters, MEMS fabrication processes, optical reasons, reliability issues, etc. They affect the design in different ways; for example, two fibres cannot be brought closer than 250μm. This constraint will affect the optical design and the overall dimensions of the device. Taking all such constraints into account, an optical design process is outlined. The effect of individual errors is studied on a key parameter like insertion loss. This gives the designer information about which parameters the design is more sensitive to and will help in deciding the manufacturing tolerances required in the different stages. Using all of this data, an "ideal design" is developed. A Monte Carlo analysis is carried out on that design to show the effect of errors occurring simultaneously. The paper concludes with a flow chart of a suggested design process to be used when designing optical MEMS devices.

We report on a new integrated optical pickup for double layer DVD's. The optics is almost integrated by means of diffractive optical elements. Dual focus as well as focal control is done by a liquid crystal cell.

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Journal of Applied Remote SensingJournal of Astronomical Telescopes Instruments and SystemsJournal of Biomedical OpticsJournal of Electronic ImagingJournal of Medical ImagingJournal of Micro/Nanolithography, MEMS, and MOEMSJournal of NanophotonicsJournal of Photonics for EnergyNeurophotonicsOptical EngineeringSPIE Reviews